The present invention relates to a system for cooling an engine when the operation of the engine is stopped under high-temperature conditions of the engine.
When a motor vehicle stops after it has run at a high speed, the heat radiated from the engine in the motor vehicle is trapped in an engine compartment when the operation of a radiator fan is stopped. The trapped heat tends to overheat fuel in fuel passages defined in a carburetor, thus casing socalled "percolation" to fill the fuel passages with evaporated fuel. When the engine is started again, the fuel vapor enters the combustion chambers where it wets the spark plugs thereby preventing an air-fuel mixture from being ignited.
Fuel pipes and a fuel pump are also heated under high underhood temperatures to produce vapor bubbles in the fuel. This phenomenon is called "vapor lock". The vapor bubbles prevent the fuel from being supplied to the engine, which therefore cannot be restarted. Where the engine is equipped with a fuel injection system, such vapor lock is also caused in fuel injection nozzles and fuel pipes coupled thereto which prevents the fuel from being injected from the nozzles with the proper timing into the combustion chambers.
One solution has been to drive a cooling fan while the engine is at a high temperature after it has been shut off. The cooling fan is started when a certain heated condition of the engine is detected. It has been customary to detect such a heated condition of the engine by detecting the temperature of cooling water in the radiator.
FIG. 6 of the accompanying drawings shows how the cooling water temperature varies with time. The graph of FIG. 6 indicates that the cooling water temperature rises to a large extent within 5 minutes right after the engine has been shut off. This temperature rise occurs because forced heat radiation from the cooling water in the radiator is no longer possible when the radiator fan is stopped. The temperature changes were plotted in FIG. 6 at an ambient temperature of 40.degree. C. after the motor vehicle had run at 120 km/h. A temperature rise of more than 10.degree. C. took place 2 minutes after the motor vehicle had stopped, and thereafter the temperature dropped.
If the cooling water temperature to start operating the radiator fan were selected to be 120.degree. C., then it would start to operate about 1 minute after the stoppage of engine operation. This operation mode of the radiator fan would not be preferable since it might surprise the driver.
The boiling point of water is 100.degree. C. at atmospheric pressure. The cooling water temperature in the engine cooling system is not substantially raised in proportion to the temperature of the engine itself, but is normally heated only up to about 130.degree. C. during the engine operation. The cooling water temperature does not vary widely in a high temperature range, and does not well reflect the heated condition of the engine.
Where the cooling water temperature to trigger the radiator fan is selected to be relatively high for a certain motor vehicle type, therefore, the radiator fails to operate stably inasmuch as it may be driven or not driven even if the heated condition of the engine remains substantially the same.
In some radiator fan control schemes the fan is stopped in operation when the cooling water temperature drops below a certain reference temperature. If the reference temperature happens to be inappropriate, however, the cooling water temperature is apt to rise again after the fan has stopped, and the fan is required to be actuated again.